An in situ, high-temperature, powder diffraction investigation was performed for iron-free clinopyroxenes with compositions Ca0.40Mg1.60Si2O6, Ca-0.52 Mg1.46Al0.05Si1.98O6, Ca0.59Mg1.41Si2O6 and Ca-0.70 Mg1.30Si2O6, up to 850degreesC using synchrotron radiation (ESRF, Grenoble). In samples with compositions Ca0.52Mg1.46Al0.05Si1.98O6 and Ca0.59Mg1.41Si2O6, evidence of for the P2(1)/c-C2/c displacive phase transition was seen in changes in lattice parameters at T approximate to 550 and 300degreesC respectively. Landau modelling of the phase transition behaviour for the sample with composition Ca0.52Mg1.46Al0.05Si1.98O6 shows a tricritical behaviour [T-c = 547(16)]. Comparison with the transition behaviour in other samples with lower Ca contents along the join diopside-enstatite indicates that a decrease in T-c, and a switch from first-order to tricritical behavior occurs with increasing Ca content. The change in the transition behaviour was related to an interaction with the antiphase domains at the nanoscale.
- clinopyroxene, in situ HT powder diffraction synchrotron radiation
- phase transition
- thermal expansion
- DIOPSIDE-ENSTATITE CAMGSI2O6-MG2SI2O6
- THERMODYNAMIC PROPERTIES